The ocean is a vast and rich world, and marine science exploration causes great concern to many scientists. Underwater acoustic communications and relevant information science and technology have an important role in promoting the development of marine industry. However, an underwater acoustic communication channel has a relatively high time-selectivity and frequency-selectivity, so the underwater acoustic channel is widely considered to be one of the most challenging communication media. Compared to a terrestrial radio channel, a typical shallow underwater acoustic channel shows good time dispersion effect, and inter-symbol interference is very serious. In this case, an effective and fast equalization algorithm is needed to eliminate serious inter-symbol interference of an underwater acoustic channel or the orthogonal frequency-division multiple access technology is used to change a selective fading channel to be a flat fading channel.
Because of the complex and changeable underwater environment, the intensity and phase of an underwater acoustic signal is not stable with random ups and downs of sea surface, as a result, multi-path time delay caused thereby is serious. For terrestrial wireless communications, the length of a channel response is generally considered to be a few symbol periods and is relatively easy to equalize. But for underwater acoustic communications, the length of a channel response is up to hundreds of symbol periods, resulting in serious inter-symbol interference. Meanwhile, the background noise power of an underwater acoustic channel is relatively large, and a signal-to-noise ratio is low. The background noises of an underwater acoustic channel mainly comprise marine environment noise and noise from ships, such as interference noise caused by wind, rain and tides, ship noise, noise from marine life, resulting in the complexity of underwater acoustic channel noises.
A transmission rate in an underwater acoustic communication system is much slower than a transmission rate in terrestrial communications, so that real-time demand is not high. But because seawater medium absorbs interference of an underwater acoustic signal and a seabed channel environment is complex and changeable, a received underwater acoustic signal is seriously distorted. So the problem that needs to be urgently solved in underwater acoustic communications is to minimize a bit error rate as much as possible, so as to ensure reliable communications. For a single-carrier underwater acoustic system, the main problem is an inter-symbol interference (ISI) problem, and symbolic equalization may be performed in a time domain to reduce inter-symbol interference. For a multi-carrier underwater acoustic system, the existing main problem is inter-subcarrier interference caused by inaccurate frequency timing, and in this case, equalization may be performed on subcarriers in a frequency domain to reduce inter-subcarrier interference. Both solutions are available in minimum BER guidelines, this patent is primarily to achieve a balanced symbol for the minimum error rate when the field of underwater acoustic communication system, its implementation can be similarly applied to underwater acoustic communication in the frequency domain.
The South China University of Technology provides an adaptive channel equalizer based on a minimum bit error rate rule and in implementation method therefor (Chinese patent number: CN 102916916 A). Main features of the present patent is that it is distinguished from a traditional adaptive equalization algorithm and is directly derived from a minimum bit error rate rule. A filtering output signal is mapped into a parameter of equalizing the degree of code errors as a basis for next time of filtering parameter modification. The patent is mainly applied to wireless channels with relatively less channel response, has a convergence speed faster than an adaptive method based on a least mean-square rule, a small amount of computation and a simple equalizer structure and is easy to implement, but it is not applicable for an underwater acoustic communication channel with long-channel response.
In addition, a dual-mode adaptive decision feedback equalization module and an implementation method therefor (Chinese patent application number: 20140041987.1) provided by the South China University of Technology proposes a decision feedback implementation method based on a minimum bit error rate. The equalization module uses a decision feedback structure and can be switched to work in two modes. The patent is suitable for an underwater acoustic channel, a time of which is unchanged or is changed very slowly and has a convergence speed faster than that of an equalizer without feedback results, but for an underwater acoustic channel of which the channel changes fast, it is difficult to converge to a desired result.
In a practical underwater acoustic communication system, an adaptive equalization module can be evaluated according to the bit error performance, convergence speed and algorithm complexity of an equalization algorithm, while the reliability of transmission of a communication system is evaluated mainly based on a bit error rate. The patent implementation method mentioned above can meet the requirements of reducing a bit error rate as much as possible, but in actual underwater acoustic communication applications, the number of times of iteration is not expected to be more, i.e. the convergence speed of an equalization module is required to be very fast in order to meet the real-time requirements in underwater acoustic communications. Meanwhile, in a time-varying underwater acoustic channel, when the convergence speed is greater than a channel response change speed, it is possible to achieve a satisfactory convergence result, so only an equalization module with fast convergence can be applied to a changeable underwater acoustic channel.
Most of the existing adaptive equalization algorithms have a lot of shortcomings, cannot take into account of reducing a bit error rate as well as having a higher convergence speed at the same time and is difficult to be applied in a complex underwater environment, so that an adaptive RLS decision feedback equalizing system for realizing a minimum bit error rate in the present invention has the capacity of fatly converging to a very low bit error rate and is thus more suitable for an underwater acoustic communication system.